The invention concerns a device for measurement of the torsion torque applied to a rotating shaft, a method to measure the torque by means of such a device, a steering column as well as a module comprising such a device.
For example, it pertains to the measurement of the torsion torque applied to a steering column of a vehicle through the intermediary of the steering wheels.
The term steering column conventionally denotes a tubular element attached to the body of the vehicle, under the dashboard, which guides and supports the transmission shaft connected to the steering wheel.
The steering wheel is then a device for manual control, which is connected to the steering wheels and used by the driver to steer the vehicle.
The invention also concerns the uncoupled steering columns. In contrast to the conventional steering columns, the uncoupled steering columns are not connected with a gear box which transforms the circular movement of the steering wheel into the angular displacement of the steering arm which causes the orientation of the wheels.
However, with such uncoupled directions, there is no direct mechanical link between the steering wheels and the wheels which are resting on the ground, whether the vehicle is an actual vehicle or belongs to a simulation system.
This simulation can pertain to a game object, connected with learning in driving schools, or it can be pertain to interactive driving simulation for the needs of car builders.
In such simulators, the compensation for the stresses at the level of the steering wheel, thanks to a mechanism which generates a resistant torque on the steering wheel as a function as the type of vehicle to be simulated, which may or may not be steered using a power steering system, must take into account the driving conditions to be recreated.
The measurement of the torque applied to the steering wheel is consequently essential to ensure a good simulation in real time.
The measurement of the torque on the shaft of the steering wheel is also very important in automated steering systems of assisted steering systems.
Indeed, triggering the assistance notably depends on the torque applied by the driver to the steering wheel.
The device for measuring the torsion torque used in automated steering systems emits a signal which indicates the orientation torque applied by the driver to the steering wheel and thus to the transmission shaft of the steering wheel of the vehicle.
This signal is conventionally addressed to a steering assistance calculator which triggers the assisted steering system, for example by controlling an electrical motor, in the case of an electrical automated steering system.
The invention can also apply to other fields such as the field of transmission of forces, for example to the wheels of the vehicle or to the control of the orientation of said vehicle by means of the measurement of the applied torque.
Devices are already known for measuring the torsion torque, which devices use stress gauges which are glued to a test body.
This type of technology presents the drawback of requiring a rotating passage which, for reasons pertaining to space requirement and reliability, is not desirable, notably for measuring the torque applied to a steering column of a vehicle.
Other technologies are also known in which one measures the angle of deformation of the test body for example through the intermediary of a change in a magnetic coupling or of a characteristic of the material placed under tension, or by measuring two input and output angles of the torque bar.
For example, the detection of the angular shift of two generators of magnetic fields with reference to detecting devices allows the delivery of an analog signal proportional to the applied torque.
This type of analog magnetic technology presents a certain number of drawbacks, in particular connected with a permanent control of the gap over one turn to preserve the measurement precision, and with the temperature compensation of the output signal which requires a differential measurement system.
Thus, in the context of the analog devices for measuring torques of the prior art, the control of the gap and of the temperature drifts complicates the assembly because of the requirement to use a more space consuming mechanical system.
Moreover, the document EP-0 284 508 describes a device for measuring the torque which uses two magnetic coders which are respectively connected in the vicinity of each end of a torque bar. In this device, the signals of the first coder are detected by a pair of sensors, while the signals of the second coder are detected by a sensor. The signals so obtained are treated by a microprocessor to obtain the applied torque.
To eliminate variations in the gap, this document proposes the use of a high pass filter, but this solution is not entirely satisfactory, because, by eliminating only the low frequency signals, it does not present a sufficient effectiveness.
In addition, the device described in the document EP-0 284 508, by determining a temporal difference between the detected signals, does not allow the obtention of the applied torque when the speed of rotation is zero.
Therefore, the invention is intended to overcome these drawbacks notably by proposing a device for the measurement of the torsion torque applied to a rotating shaft whose output signal can be rendered independent of, on the one hand, the amplitude of the read magnetic field and thus of the variations in the gap, and, on the other hand, the decrease in the magnetic field due to the temperature, while, at the same time, being reliable and adapted to a severe environment of the automobile type.
Thus, it makes it possible to work in a broad gap range and it avoids a differential measurement to compensate for the temperature drifts.